JPS61230042A - Diagnosing method for corrosion of steel structure - Google Patents

Abstract

PURPOSE:To diagnose the present corrosion state and the remaining service life of a steel structure by measuring the remaining wall thickness of each part of the steel structure with an ultrasonic thickness meter and measuring the corroding speed of the same part with a corroding speed detector. CONSTITUTION:The corroding speed detector 2 of metal material is fixed on a steel pipe pile 3 used for the structure, for instance, a lading pier by a jig 4 and performs the automatic measurement by a program indication from a measuring system 1 and the arithmetic processing and figuring are performed by a calculation processing machine 5. The detector 2 catches a corrosion reaction of the metal as the impedance at the interface and calculates the reaction electric current quantity from the relation with the impressed voltage and converts it into the corroding speed. Then, first, the precise measurement of the remaining wall thickness is performed every four places in a circumferential direction of the steel pipe pile 3, for instance, with a 50cm pitch of the depth of water by the ultrasonic thickness meter and the corroding speed of the same place at the point of time is measured by the detector. Then, the necessary wall thickness for every depth of water is calculated from the designed load or the like to calculate the remaining life.

Description

Detailed Description of the Invention (Field of Industrial Application) The present invention is applicable to corrosion diagnosis of steel structures, particularly civil engineering and architectural structures used in contact with solutions containing electrolytes such as seawater and river water. It is about the method.

(Conventional technology and problems) Steel structures made of metal materials that are used in contact with solutions containing electrolytes such as seawater or river water are susceptible to damage as a structure due to holes and a reduction in cross section due to corrosion of the material. Functionality may be lost. On the other hand, as we enter a period of stable growth, there is a social demand for extending the life of equipment. Therefore, there is an increasing need for corrosion diagnosis to determine the current state of corrosion of equipment and structures and how much remaining life they have.

The currently used corrosion diagnosis methods include: (1) Small specimens for corrosion monitoring are installed in advance when new equipment or structures are installed, and the specimens are inspected periodically to estimate remaining service life from corrosion trends. how to.

(2) A method of measuring the remaining wall thickness of the pitting area at the time of conducting the corrosion investigation, estimating the maximum amount of wall thinning using extreme value statistical methods, and determining whether the service life has expired based on the relationship with the minimum thickness required for functionality. .

(3) For example, Port and Harbor Engineering Research Materials No. 413.1982
Using the corrosion rate calculation method described in 2.5,
Find the average corrosion rate by dividing the difference between the remaining wall thickness and the initial wall thickness of the structure by the period of use, and then divide the difference between the minimum wall thickness required for structural strength and the remaining wall thickness by the average corrosion rate. How to calculate remaining life.

(4) Measuring the change in wall thickness at the same location after a period long enough for the change in wall thickness due to corrosion to be measurable, and calculating the average corrosion rate from the amount of change and period, and determining the minimum thickness required for structural strength. A method of determining remaining life by dividing the difference between wall thickness and remaining wall thickness by the average corrosion rate.

etc.

(1) is only corrosion information regarding small pieces for monitoring, and does not represent the condition of the equipment or structure itself. (2) is effective for predicting the lifespan of pipelines that lose their functionality due to pitting corrosion, but is difficult to apply in cases where overall corrosion affects the lifespan, such as structures. Regarding (3), in most cases the initial wall thickness of the structure is not measured in advance, and in this case catalog data must be used.
525), an error occurs in calculating the average corrosion rate because a tolerance is allowed in the plate thickness. Also, changes in the usage environment (
The accuracy of corrosion diagnosis is low because the corrosion rate changes mainly due to dissolved oxygen) and the application of cathodic protection. (4
) requires a long time for diagnosis and is not practical because results cannot be obtained in a short time.

(Problems to be Solved by the Invention) The present invention is based on the current corrosion rate determined by directly measuring the target object on-site, without using corrosion information on small specimens for monitoring or data on initial wall thickness. The present invention provides a method for quickly and accurately diagnosing corrosion of civil engineering and architectural structures using steel, based on the results of measurements of the remaining wall thickness.

(Means for Solving the Problems) The present invention makes it possible to directly measure the residual wall thickness of civil engineering construction structures using steel, for example, using an ultrasonic thickness needle, and the corrosion rate on site. The present invention aims to improve the accuracy of corrosion diagnosis by using the current corrosion rate determined using a device such as a corrosion rate detector for metal materials (Japanese Patent Application No. 174169/1982). The corrosion rate detector for metal materials detects the range specified by the device between the metal and the polarizing electrode.
Measure polarization resistance by uniformly polarizing the material using the AC impedance method, constant current method, coulostatic paros method, constant current double waveform method (dual frequency method), etc., and convert it into a corrosion rate. It is something.

Figure 1 shows the basic concept of corrosion diagnosis.

The horizontal axis of the figure shows time, and the vertical axis shows the wall thickness of the structural member.

Other symbols are: Tt: @at the time of corrosion diagnosis, tr: remaining life determined from Ts corrosion rate, tml: remaining life determined from average corrosion rate when wall thickness has a negative tolerance, tm2: average corrosion Remaining life when the wall thickness has a plus tolerance, calculated from the speed, δi: Initial wall thickness, δ11: Initial wall thickness when the wall thickness has a minus tolerance, δ12: When the wall thickness has a plus tolerance. Initial wall thickness in case, δt: Remaining wall thickness at corrosion diagnosis, δn: Minimum wall thickness required for structural strength, vt: Corrosion rate at corrosion diagnosis, Vml: Average corrosion rate determined using δil, Vm2: The average corrosion rate determined using δ12 is shown below. A feature of this method is that it does not require information regarding the initial wall thickness, but for comparison with conventional methods,
The initial wall thickness and average corrosion rate are also displayed.

Residual wall thickness δt and corrosion rate Vt at time of diagnosis Tt
From the relationship between the measurement results of To give an example in detail, at the time Tt when performing the diagnosis, measurements are taken at two or more n points (i = 1 to n) in the circumferential direction of the structure to be diagnosed at the same water depth.
)L, residual thickness δt (hl i and corrosion rate Vt (h
) Estimate the remaining lifespan tr of the structure from the relationship with the minimum wall thickness δn (hl) calculated from the measurement results of i and the structural analysis performed separately. Measure at 3 to 5 points around and average.

Further, in terms of structural analysis, the required minimum wall thickness may be different in two directions perpendicular to the water depth direction, but for corrosion diagnosis, the smaller one may be set as δn (h). ) First, the remaining thickness δ
t (h) i and corrosion rate v t (h+ each circumferential average δt (h) taean and V t (h)
means and the maximum and minimum difference in corrosion rate V t
(h) w as δt (h) mean−Σδt (h
) i/nV t (h) sean=ΣV t (
h) i/nV t(h1w=V t(h) i
It is calculated as 1llax-V t(h) i II+in.

Next, determine the velocity distribution V t (h) in the water depth direction as
(h) Calculate as mean-δn (h) / V t (h). Corrosion is an electrochemical reaction at the interface, so it is not a fixed thing, but is influenced by changes over time such as dissolved oxygen concentration, and when captured as an instantaneous value, the corrosion rate is the maximum Q,
There are daily and seasonal changes of about 1m/year. Therefore,
For V t (h)WS2.1/year, the average value is adopted.

If V t (h) w≧0.2 m/year, it means that the corrosive environment is severe, such as in a tidal zone or a splash zone, and corrosion has progressed in some areas, so repeat the measurement. Then, from the viewpoint of safety, the maximum value of the corrosion rate is taken as the corrosion rate of that part.

Finally, from the minimum value of tr(h), the lifetime tr of the structure is determined as tr=tr(h)n+in, and the structurally critical portion (h) is determined. Vt
(If hew≧0, 24wm/year exists, that part is re-examined as a caution area.

〔Example〕

Hereinafter, a detailed explanation will be given based on a specific example.

Figure 2 shows an example of corrosion diagnosis for steel pipe piles used in piers. A metal material corrosion rate detector 2 is fixed to a steel pipe pile 3 to be diagnosed using a jig 4, automatic measurement is performed according to program instructions from a measurement system 1, and arithmetic processing and diagramming are performed by a computer processor 5. 6 is an ultrasonic thickness gauge for measuring the remaining wall thickness of the steel pipe pile. The corrosion rate detector 2 captures the corrosion reaction of metal as impedance at the interface, calculates the amount of reaction current from the relationship with the applied voltage, and converts it into a corrosion rate.

First, four locations in the circumferential direction of the steel pipe pile (in the case of n-4)
The remaining wall thickness was precisely measured using an ultrasonic thickness needle at a pitch of 50 CII in the water depth direction, and the corrosion rate at that point in time was measured using a metal corrosion rate detector.

Next, from the distribution of the moment M (h) calculated from the design load, the axial force N (hl) in the water depth direction (h), and the allowable stress σ of the material, the required wall thickness for each water depth is determined by the following relational expression: Seek.

Here, A (h): Cross-sectional area of Ii pipe pile, Z (h)
: Section modulus of steel pipe pile, D (h): Outer diameter of steel pipe pile, d (
h): m is the inner diameter of the pipe pile, π: pi. Since there are 8 types of variables, 4 types of given conditions, 4 types of unknowns, and 4 types of relational expressions, the above relational expression can be solved analytically.

Figure 3 shows the measurement results. The vertical axis shows the water depth, and the horizontal axis shows the wall thickness of the steel pipe pile. The broken line shows the minimum wall thickness necessary for structural strength, and the solid line shows the distribution of the average remaining wall thickness in the circumferential direction in the water depth direction. Although the required minimum thickness is satisfied as a whole, corrosion near the mean low tide surface is significant, and δt (hl mean−δ
From n(h)=1m, it can be seen that the margin is one day, and this area determines the life of the pier.

Corrosion diagnosis is mainly required 10 to 20 years after construction, and the average corrosion rate from the average high tide surface to the seabed is 0 lines, which is said to be commercial Q, l tm / year. The case goes beyond this and requires caution.

Figure 4 shows the distribution of corrosion rates in the water depth direction as measured by a metal corrosion rate detector. In the figure, the X mark is circumferential direction 4
The average corrosion rate at a point, and the arrows indicate the corrosion rate distribution at four points in the circumferential direction.

From Equation 1, the method for determining the corrosion rate distribution in the water depth direction, remove h = 0.5 m, V t (h) =
V t (hl tocean. h = 0.5
For m, V t (h) w>0.2 crane/ye
Since ar, Vt (0,5) = Vt (0,5)
Adopt i max = 0.55 tm/year. In FIG. 5, tr(hl=(δt (hl mea
n-δn (hl / V t (shows the life distribution obtained from hl. The life of the structure is determined by h = 0.5 m, and from tr = 1.010.55 = 1.82, the remaining life is It was less than 2 years old and was in a condition that required immediate repair work.

For port structures that use steel sheet piles, steel pipe sheet piles, and other steel structural materials other than steel pipe piles, this also applies to those structural materials.
A similar diagnosis can be made by constructing a formula.

〔Effect of the invention〕

According to the present invention, the remaining wall thickness of the member is measured using an ultrasonic thickness needle, micromake, etc., and the corrosion rate is measured using a device that can directly measure the corrosion rate on site, such as a corrosion detector for metal materials. Corrosion diagnosis of civil engineering and construction steel structures can be performed quickly and accurately. In addition, corrosion supply and disconnection can be performed without using uncertain information such as information on initial wall thickness that includes standard errors, changes in the corrosive environment from construction to corrosion diagnosis, and temporary validity of cathodic protection. It can be used for maintenance of steel structures, etc.
In turn, it becomes possible to improve the safety and extend the life of steel structures.

[Brief explanation of the drawing]

Figure 1 is an explanatory diagram of the significance of the corrosion rate at that point used in corrosion diagnosis, Figure 2 is an explanatory diagram showing an example of corrosion diagnosis of structures, and Figures 3.4.5 are This is an example of a chart used when conducting corrosion diagnosis. In the drawing, 1 is a measurement system, 2 is a metal material corrosion rate detector, 3 is a steel pipe pile, 4 is a jig, 5 is a computer processor, and 6 is an ultrasonic thickness needle. Applicant Nippon Steel Corporation Patent Attorney Minoru Aoyagi Fig. 1 1, Measurement system 2, Corrosion rate suppression of metal materials J:, 3, it!l pipe 4, 5 equipment 5, Computer processor 6, a sound wave f# meter Figure 2 Procedural amendment (voluntary) May 13, 1985 Manabu Shiga, Commissioner of the Patent Office, 1985 Patent Application No. 71829 2 Name of the invention Steel structure Corrosion diagnosis method & relationship with the person making the amendment case Patent applicant address 2-6-3 Otemachi, Chiyoda-ku, Tokyo Name (6
65) Nippon Steel Corporation Representative Takeshi 1) Yutaka 4, Agent 〒101 5, Date of amendment order None 6, Number of inventions increased by amendment None 7, Subject of amendment”\ / ( 1) r tr(h) = (δt(h)mean-δn(h
)/Vl(h) J rtr(h)=(δt(h) m
ean-δn(h))/Vt(h)J. 0) Correct "commerce" on page 1, line 8, to "normal".

Claims (1)

[Claims] The remaining wall thickness of each part of a steel structure is measured using an ultrasonic thickness meter, and the corrosion rate of the same part is measured on-site using a corrosion rate detector, and the remaining life of the steel structure is estimated from the results of both measurements. A method for diagnosing corrosion of steel structures.